Motor controls of the type considered herein are generally well known and documented. See, for example, the General Electric publication entitled "Electronics Data Library--Semiconductors; Transistors--Diodes" (1984).
Generally, such motor controls vary the speed of a three-phase induction motor by altering either the voltage, the frequency, or both, of the drive signal applied to the stator windings of the motor. In such applications, it is desirable to protect both the motor and the control from extreme values of voltage and/or current. One such known method is to include a pair of current sensors, each being situated in series with a motor winding. When extreme motor currents are detected by either sensor, the motor can be shut down to prevent damage to it. Although this technique works well, its drawback resides in the requirement for at least two current sensors.
It is also desirable to isolate the motor's control logic and drive circuits from the AC line voltage (i.e., he AC "mains") and to protect the motor and the control from extreme variations in the amplitude of the AC line voltage. Such isolation and protection are conventionally provided by using a transformer or optical coupler to isolate the control from the AC line voltage. An additional transformer is used to sense extreme variations in the AC line voltage so that the control and the motor can be protected when such extreme variations occur. For economy, however, it is desirable to achieve the same results without the use of the additional transformer.